Patent Application
20150025562
The devices and methods described below relate to the field of treatments for aneurysms and blood clots.
Intracranial aneurysms and clots can be treated with implantable devices such as flow diversion tubes, embolization devices, aneurysm bridging devices and scaffolding devices and thrombectomy devices. These devices are delivery through a microcatheter which is first delivered into an intracranial artery affected by an aneurysm or clot. Previously, these devices were preloaded onto the tips of microcatheters for introduction into a patient. More recent devices are not preloaded but are instead loaded via a proximal hub of the microcatheter, after the microcatheter has been placed with its distal tip deep in the intracranial vasculature, near the site of the aneurysm or blood clot. Examples of self-expanding devices delivered through microcatheters include neurovascular remodeling devices such as Covidien's Pipeline® embolization device (which is a braided tube placed to span and bypass an aneurysm) and Covidien's Solitaire® thrombectomy device, which is used to retrieve clots from intracranial arteries, and our own aneurysm bridging device which is used as a scaffold to hold embolic material within an aneurysm. These device are passed through microcatheters in a compressed state, and released from the microcatheter to expand to a much larger diameter. The compressed shape exerts an outward radial force, which causes friction with the inner wall of the microcatheter and makes delivery difficult. As the devices are forced through the microcatheter, they may scrape the inner wall of the microcatheter and shave off small pieces of the lining, which may then be forced from the microcatheter and deposited in the brain. The FDA's MAUDE Adverse Event Report Database includes reports of liner damage, excess friction, and skiving for various catheters used for delivery of expandable devices.
The devices and methods described below provide for a easier delivery of expandable vascular reconstruction devices (such as flow diversion tubes, embolization devices, aneurysm bridging devices, scaffolding devices and thrombectomy devices) into the vasculature of a patient, through an improved microcatheter. The improved microcatheter is constructed with a liner that has a low coefficient of friction and increased toughness vis-à-vis typical PTFE or ePTFE liner. The low coefficient of friction reduces the delivery friction between the microcatheter and expandable devices that are passed through the microcatheter, and reduces skiving of liner material that might be deposited into the vasculature.
The microcatheter is enhanced in that the microcatheter includes a stretched PTFE (polytetra-fluoroethylene) liner. The PTFE liner (or a portion of the liner) is only stretched longitudinally and not circumferentially or radially over at least a portion of the length of the microcatheter. The enhanced liner results in a reduction of the delivery friction and improved toughness and resistance to scraping. Preferably, the microcatheter comprises a layered construction, including a coil disposed over the liner and a sheath re-flowed over the coil. The microcatheter can alternatively include additional layers or it may consist merely of a stretched PTFE liner without additional layers, and the number of layers can vary along the length of the microcatheter
The microcatheter can be used to deliver aneurysm treatment devices, embolization devices, neurovascular remodeling devices and thrombectomy devices that are loaded via a proximal hub of the microcatheter and then navigated the entire length of the microcatheter before deployment at the desired target site in within the carotid artery.
The layers of the proximal segment of the microcatheter are illustrated in
To form the microcatheter, the PTFE liner is heated and stretched longitudinally, but is not stretched circumferentially or radially. A distal segment of the liner is then wound with a single layer of nitinol coil of varying gaps (as shows in
Though many methods may be used to stretch the PTFE liner and assemble the microcatheter, the following method may be used. To stretch the PTFE liner longitudinally, while avoiding circumferential or radial stretching, a PTFE tube of appropriate internal diameter, outer diameter and length is pulled and heated to stretch the tube and draw the inner diameter down on a wire with a diameter corresponding to the desired internal diameter of the liner. Thus, a nitinol wire (covered with a PTFE tube, referred to as a beading), is threaded through a PTFE tube, and the wire and tube together are held vertically, suspended from a clamp, and weights are secured to the PTFE tube to apply tension to the tube, and the PTFE tube is heated. The tube is heated and tensioned until stretched 25 to 100%, so that its stretched length is 25 to 100 percent longer than its original length (the inner wall diameter is also drawn down to about 25 to 50% of its original wall thickness). For example, for a PTFE liner of 0.021″ (0.53 mm) inner diameter, a 0.00075″ (0.02 mm) wall thickness, and 70 inch (178 cm) in length, a PTFE tube of 0.027″ (0.69 mm) inner diameter, 0.001-0.0015″ (0.0254-0.0381 mm) wall thickness, and initial length of 52 inch (132 cm) can be suspended from a clamp at one end, and weights totally about 115 grams may be secured at the other end, and the PTFE tube can be heated with a heat gun to achieve the PTFE liner of 0.021″ diameter and stretched length of 70 inch (178 cm), with a inner wall diameter drawn down to 0.00075″ (0.02 mm). Thus, the PTFE tube in this example is stretched in the longitudinal dimension by about 40%. The inner diameter of the tube is drawn down, in this example, from 0.027″ to 0.021″ (or about 20-25% of its initial diameter) as well, in response to the longitudinal stretching, but this draw-down, while beneficial in this embodiment, need not be achieved to obtain the benefits of the stretched liner. The heat gun in this example is used to soften the PTFE tube. The heat gun can be drawn over the PTFE tube at constant speed, or it may be drawn over the PTFE tube at different speeds over different longitudinal regions, to create different degrees of stretching, for example to stretch a distal region more than a proximal region (this will result in a more flexible distal region with a thinner wall). Proximal and distal segments of the PTFE heat shrink tube can also be heated differentially in any suitable manner. The PTFE tube may be heated by oven or other techniques. After the PTFE tube is drawn down to the wire, the coil covering the distal segment may be wound over the distal segment, and cross-wound coils covering the proximal segment may be wound over the proximal segment (the proximal cross-wound coils and the distal coil are then welded together at their junction). The outer sheath is applied by slipping pebax tubes and Nylon tubes of appropriate composition and hardness over the distal coil and proximal cross-wound coils, and covering this outer sheath tubing with heat shrink tubing, and heating this assembly to re-flow the pebax and Nylon tubing and compress it into the distal coil and proximal cross-wound coils. The heat shrink tubing used to compress and re-flow the outer sheath material is removed from the three layer microcatheter body and discarded, and the nitinol wire is removed from the stretched PTFE liner, and the three-layer microcatheter body is trimmed (at the distal and proximal ends, as necessary) and secured to a proximal hub or Luer fitting. Radiopaque markers can be applied over the outer sheath, or embedded or sandwiched between layers of the microcatheter, to enhance visualization of the catheter when viewed fluoroscopically during use within the body.
In use, a guide catheter is introduced into the femoral artery via a small incision in the groin of a patient. The guide catheter is navigated from the femoral artery 21 through the aorta 22 and advanced into the carotid artery 23. The ultimate target site may be deep within the intracranial vasculature, perhaps within the Circle of Willis 16 or even deeper. The enhanced microcatheter is introduced through a guide catheter and delivered to the target location in the intracranial vasculature. The expandable vascular reconstruction device is then pushed through the microcatheter to the target site within the intracranial vasculature.
The microcatheter has been described above as a three-layer construction. The microcatheter can consist of the three layers, without additional layers, and this configuration greatly facilitates device delivery because the liner is more lubricious vis-à-vis an un-stretched PTFE tube. Also, the advantage of the stretched liner can be achieved in a microcatheter that includes only the enhanced liner. Specifically, a microcatheter, or a distal segment of the microcatheter, that consist of only the stretched PTFE liner as disclosed above, without additional layers disposed over the PTFE liner. The stretched liner can span the entire length of the microcatheter or may be stretched only on a portion of the microcatheter, such as the distal segment where resistance to passage of the device is greatest.
The devices above typically exert an outwardly directed radial force in excess of 0.075 Newtons per millimeter of length. The liner, and especially the inner surface of the liner, is modified to reduce friction between the liner and the device which is pushed through the catheter, such that the longitudinally directed force required to push the device through the microcatheter is less than 380 grams while distal region of the catheter is located within the Circle of Willis.
Overall, compared to an un-stretched liner, the stretched liner results in a microcatheter with a 40% reduction in friction and the force required to push devices through tortuous regions of the microcatheter, and a 20% reduction of the radial coefficient of friction of the internal surface of the tube. The coefficient of friction as used here refers to a coefficient defined by a test unique to expandable vascular reconstruction devices and microcatheters. This test entails determining the outward radial force (in Newtons) of the expandable vascular reconstruction devices when compressed to match the inner diameter of the microcatheter, and measuring the force (in Newtons) required to pull the expandable vascular reconstruction devices through the microcatheter (while in contact with the liner). The radial coefficient of friction is defined here as the ratio of the radial force exerted by the device to the force required to pull the device through the microcatheter. Under this test, the radial coefficient of friction measured between the expandable vascular reconstruction device and the inner surface of the PTFE liner is in the range of about 0.07 to 0.25. We refer to this coefficient of friction as a “radial” coefficient of friction to distinguish it from the coefficient of friction for PTFE obtained under ASTM D1894, which for PTFE is typically 0.02 to 0.1.
The modified liner is also stronger and tougher than initial PTFE tube. The PTFE liner obtained through the process of stretching described above has a Young's modulus of over 100 Kpsi, typically about 117 Kpsi, which is much higher than the typical Young's modulus (71 Kpsi) of the stock PTFE tubing. The PTFE liner also has greatly increased tensile strength, increased 50 to 100%, from the unmodified PTFE tube, with a tensile strength improved from about 18 kpsi to over 20 kpsi, typically about 28 kpsi.
While the preferred embodiments of the devices and methods have been described in reference to the environment in which they were developed, they are merely illustrative of the principles of the inventions. The elements of the various embodiments may be incorporated into each of the other species to obtain the benefits of those elements in combination with such other species, and the various beneficial features may be employed in embodiments alone or in combination with each other. Other embodiments and configurations may be devised without departing from the spirit of the inventions and the scope of the appended claims.
